Redesign Cob Spec to be easier to use.

This is a major redesign to how cob spec works.
- Updated fitnesse to the latest version.
- Combined tests into a single suite.
- Removed duplication in fixture code.
- Updated tests to new fixture code.
- Updated .rvmrc to 1.9.3
- Removed Rubyslim submodule because it did not work with 1.9.3.
- Updated README with new instructions.

There are probably some other minor changes that I've forgotten.